Ultrasonic holography detector

ABSTRACT

There is disclosed an improved ultrasonic hologram or other ultrasonic imaging process that accurately forms phase and amplitude information of the hologram in a manner that renders the unit relatively insensitive to environment vibrations, and provides long maintenance free functioning lifetime. Specifically, there is disclosed an improved ultrasonic hologram detector component that forms an ultrasonic hologram on the surface of a detection deformable detector material, resulting from the deformation of the surface. The surface deformation is due to the reflection of an ultrasound (ultrasonic) energy profile of the combination of an “object wave” that passes through an object and that of a “reference wave” that is directed to the surface at an off axis angle from the “object wave”.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/589,863 filed Jun. 8, 2000, now U.S. Pat. No. 6,353,576, andcontinuation-in-part of U.S. patent application Ser. No. 10/053,249filed Jan. 15, 2002, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention provides an improved ultrasonic holography orother ultrasonic imaging process that accurately forms the phase andamplitude information of the hologram in a manner that renders the unitinsensitive to environment vibrations, and provides long maintenancefree functioning lifetime. Specifically, the improved ultrasonichologram detector component forms an ultrasonic hologram on the surfaceof a deformable detector material that results from the deformation ofthe surface. This is due to the reflection of an ultrasound energyprofile of a combination of an “object wave” that passes through theobject and that of a “reference wave” that is directed to the surface atan off axis angle from the “object wave”.

2. Description of the Related Art

The central element field of holography is fulfilled by combining orinterfering an object wave or energy with a reference wave or energy toform an interference pattern referred to as the hologram. A fundamentalrequirement for the forming of the hologram and the practice ofholography is that the initial source of the object wave and referencewave or energy are coherent with respect to the other wave. That is tosay, that all parts of both the object wave and the reference wave areof the same frequency and of a defined orientation (a fixed spatialposition and angle between the direction of propagation of the twosources). When performing holography the object wave is modified byinterference with structure within the object of interest. As thisobject wave interacts with points of the object the three-dimensionalfeatures of the object impart identifying phase and amplitude changes onthe object wave. Since the reference wave is an unperturbed (pure)coherent wave, its interference with the object wave results in aninterference pattern which identifies the 3-D positioning andcharacteristics (ultrasonic absorption, diffraction, reflection, andrefraction) of the scattering points of the object.

A second process, (the reconstruction of the hologram) is then performedwhen a coherent viewing source (usually light from a laser) istransmitted through or reflected from the hologram. The hologram patterndiffracts light from this coherent viewing or reconstructing source in amanner to faithfully represent the 3-D nature of the object, as seen bythe ultrasonic object wave.

To reiterate, to perform holography coherent wave sources are required.This requirement currently limits practical applications of the practiceof holography to the light domain (e.g., a laser light) or the domain ofacoustics (sometimes referred to as ultrasound due to the practicalapplication at ultrasonic frequencies) as these two sources arecurrently the only available coherent energy sources. Thus, furtherreferences to holography or imaging system will refer to the throughtransmission holographic imaging process that uses acoustical energiesusually in the ultrasonic frequency range. In the practice of ultrasoundholography, one key element is the source of the ultrasound, such as alarge area coherent ultrasound transducer. A second key element is theprojection of the object wave from a volume within the object (theultrasonic lens projection system) and a third is the detector andreconstruction of the ultrasonic hologram into visual or useful format.

Although other configurations can be utilized, a common requirement ofthe source transducers for both the object and reference waves is toproduce a large area plane wave having constant amplitude across thewave front and having a constant frequency for a sufficient number ofcycles to establish coherence. Such transducers will produce thisdesired wave if the amplitude of the ultrasound output decreases in aGaussian distribution profile as the edge of the large area transduceris approached. This decreasing of amplitude reduces or eliminates the“edge effect” from the transducer edge, which would otherwise causevarying amplitude across the wave front as a function distance from thetransducer.

In the process of through transmission ultrasonic holographic imaging,the pulse from the object transducer progresses through the object, thenthrough the focusing lens and at the appropriate time, the pulse ofultrasound is generated from the reference transducer such that theobject wave and reference wave arrive at the detector at the same timeto create a interference pattern (the hologram). For broad applications,the transducers need to be able to operate at a spectrum or bandwidth ofdiscrete frequencies. Multiple frequencies allow comparisons andintegration of holograms taken at selected frequencies to provide animproved image of the subtle changes within the object.

A hologram can also be formed by directing the object wave through theobject at different angles to the central imaging axis of the system.This is provided by either positioning or rotating the object transduceraround the central axis of transmission or by using multiple transducerspositioned such that the path of transmission of the sound is at anangle with respect to the central axis of transmission.

With a through-transmission imaging system, it is important to determinethe amount of resolution in the “z” dimension that is desirable andachievable. Since the holographic process operates without limits ofmechanical or electronic devices but rather reconstructs images fromwave interactions, the resolution achievable can approach thetheoretical limit for the wavelength of the ultrasound used. However, itmay be desirable to limit the “z” direction image volume so that one can“focus” in on one thin volume slice. Otherwise, the amount ofinformation may be too great. Thus, it is of value to develop a meansfor projecting a planar slice within a volume into the detector plane.One such means is a large aperture ultrasonic lens system that willallow the imaging system to “focus” on a plane within the object.Additionally, this lens system and the corresponding motorized, computercontrolled lens drive will allow one to adjust the focal plane and atany given plane to be able to magnify or demagnify at that z dimensionposition.

The image is detected and reconstructed at the detector. Standardphotographic film may be used for the recording of light holograms andthe 3-D image reconstructed by passing laser light through the film orreflecting it from the hologram pattern embossed on the surface of anoptical reflective surface and reconstructing the image by reflectinglight from the surface. However, there is no equivalent “film” materialto record the intricate phase and amplitude pattern of a complexultrasonic wave. One of the most common detectors uses a deformabledetector material-air surface or interface to record, in a dynamic way,the ultrasonic hologram formed. The sound energy at the frequency ofultrasound (above range of human hearing) will propagate with littleattenuation through a liquid (such as water) but cannot propagatethrough air. At these higher frequencies (e.g., above 1 MHz) theultrasound will not propagate through air because the wavelength of thesound energy is so short (λ(wavelength)=v(velocity)/f(frequency)). Thedensity of air (approximately 0.00116 g/cm³) is not sufficient to couplethese short wavelengths and allow them to propagate. On the other handthe density of a liquid (e.g., water) is a favorable media to couple andpropagate such sound. For example, the velocity of sound in air isapproximately 330 meters/second whereas in water it is approximately1497 meter/second (room temperature). Thus, for water, both the density(1 g/cm³) and the wavelength (˜1.48 mm at 1 MHz) are significantly largesuch that ultrasound can propagate with little attenuation. Whereas, forair both the density (0.00116 g/cm³) and wavelength (0.33 mm at 1 MHz)are sufficiently small such that the energy at these ultrasonicfrequencies will not propagate.

Thus, when ultrasound propagating in a deformable detector materialencounters a deformable detector material-air interface the entireamount of the energy is reflected back into the deformable detectormaterial. Since ultrasound (or sound) propagates as a mechanical forceit is apparent that the reflection (or changing direction ofpropagation) will impart a forward force on this surface of thedeformable detector material air interface. This force, in turn, willdistort the surface of the deformable detector material. The amount ofsurface distortion will depend upon the amplitude of the ultrasound waveat each point being reflected and the surface tension of the deformabledetector material. Thus, the pattern of the deformation is the patternof the phase and amplitude of the ultrasonic wave.

It is in this manner that a deformable detector material-air interfacecan be commonly used to provide a near real-time recorder (“filmequivalent”) for an ultrasonic hologram. The shape of the surfacedeformation on this deformable detector material-air detector is therepresentation of the phase and amplitude of the ultrasonic hologramformed by the interference of the object and reference ultrasonic waves.

The greatest value of the ultrasonic holographic process is achieved byreconstructing the hologram in an usable manner: usually in light, tomake visible the structural nature of the initial object. In the case ofa deformable detector material-air interface, the reconstruction toachieve the visible image is accomplished by reflecting a coherent lightfrom this deformable detector material-air surface. This is theequivalent process to reflecting laser light from optically generatedhologram that is embossed on the surface of a reflecting material (e.g.,thin aluminum film).

The reflected light is diffracted (scattered) by the hologram todiffracted orders, each of which contains image information about theobject. These diffracted orders are referred to as ±n th orders. Thatpart of the reconstructing light that does not react with the hologramis referred to as zero order and is usually blocked so that the weakerdiffracted orders can be imaged. The higher the diffracted order thegreater the separation angle from the zero order of reflected light.

Once reconstructed, the image may be viewed directly, by means of avideo camera or through post processing.

Ultrasonic holography as typically practiced is illustrated in FIG. 1. Aplane wave of sound (1 a) (ultrasound) is generated by the object (largearea) transducer (1) (U.S. Pat. No. 5,329,202). The sound is scattered(diffracted) by structural points within the object (2). The scatteredsound is from the internal object points that lie in the focal plane (2a) are focused (projected) into the ultrasonic hologram plane (6). Thefocusing takes place by use of ultrasonic lens (3) (U.S. Pat. No.5,235,553) which focuses the scattered sound into a hologram detectorsurface (6) and the unscattered sound into a point (4). The lens systemalso allows the imaging process to magnify the image or change focusposition (U.S. Pat. No. 5,212,571). Since the focus point of theunscattered sound (4) is prior to the holographic detector plane (6),this portion of the total sound again expands to form the transparentimage contribution (that portion of the sound that transmitted throughthe object as if it were transparent or semitransparent). In such anapplication, an ultrasound reflector (5) is generally used to direct theobject sound at a different angle (preferably vertically to allow forthe holographic detector to have a surface parallel to ground to avoidgravity effects), thus impinging on horizontal detector plane usuallycontaining a deformable detector material which is deformed by theultrasound reflecting from the deformable detector material-airinterface. When the reference wave (7) and the object wave aresimultaneous reflected from this detector, the deformation of thedeformable detector material-air interface is the exact pattern of theultrasonic hologram formed by the object wave (1 a combined with 2 a)and the “off-axis” reference wave (7).

This ultrasonic hologram formed in the holographic detector (6) issubsequently reconstructed for viewing by using a coherent light source(9), which may be passed through an optical lens (8), and reflected fromthe holographic detector surface (U.S. Pat. No. 5,179,455). Thisreflected coherent light contains two components. These are A: The lightthat is reflected from the ultrasound hologram which was not diffractedby the ultrasonic holographic pattern which is focused at position (10)and referred to as undiffracted or zero order light; and B: The lightthat does get diffracted from/by the ultrasonic hologram is reflected atan “off-axis” angle from the zero order at position (11) and referred toas the “first order” image view when passed through a spatial filter(12). It is noted that this reconstruction method produces multiplediffraction orders each containing the ultrasonic object information.Note also both + and − multiple orders of the diffracted image arepresent and can be used individually or in combinations to view theoptical reconstructed image from the ultrasonically formed hologram bymodifying the spatial filter (12) accordingly.

The practice of ultrasonic imaging is in the industrial or hospitalsettings where insensitivity to vibrations and long term stableoperations are important to the successful use of the system. It isfurther a requirement that the detector method be able to image subtlestructures within the object being imaged and to often provideindividual frames on images as fast as 120 per second. This need isparticularly strong, for example, for breast cancer screening techniquesthat now utilize invasive mammography (providing the patient with a doseof radiation from X-Ray imaging) and yet do not have high quality imagesthat lend a sense of three dimensional structure to breast tissue.

Therefore, there is a need in the art to improve resolutioncharacteristics of transmissive ultrasonic imaging, ability to operatein environments experiencing vibrations and to perform over extendedperiods of time without service or degradation of image quality.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an ultrasonic hologram detector apparatuscomprising:

a rigid housing component describing a cavity defined by a floorcomponent composed of rigid solid material having an upper surfaceforming a first plane and lower surface forming a parallel second plane,and rigid side elements attached to the base.

In one embodiment, the cavity defines an enclosed space of a dimensionof the upper surface of the floor component and the rigid side elementsof from about 1 cm to about 5 cm in height, wherein the distance betweenthe upper surface and the lower surface of the floor component is fromabout 5 mm to about 7.5 mm.

The detector may also comprise a layer of deformable detector materialcontained within the cavity. In one embodiment, the deformable detectormaterial has a small thickness (e.g., about 0.2 mm to about 0.50 mm)whereby surface or horizontal vibration waves at the frequenciesexperienced in buildings cannot be propagated. In an exemplaryembodiment, the thickness of the detection deformable detector materialwithin the cavity is a multiple of ¼ wavelengths at the frequency beingused for imaging, whereby internal reflections within the detectordeformable detector material are minimized.

The deformable detector material may have a surface tension sufficientlysmall such that the surface will respond to fractions of a wavelength ofultrasound at frequencies greater than 1 MHz. (e.g., about 12 dynes/cmto about 19 dynes/cm), and a Kinematic viscosity such that thedeformable detector material will come to a quiescent condition within1/120 of a second or less wherein the deformable detector material(e.g., 1 cs to about 20 cs).

In another embodiment, the detector comprises an inert gas filling aspace in the cavity above the detection deformable detector material.

In an exemplary embodiment, the inert gas is selected from the groupconsisting of nitrogen, helium, argon, and combinations thereof.

In another embodiment, the ultrasonic hologram detector furthercomprises a top of the cavity composed of optically transparentmaterial. The optically transparent material may be glass and, in anexemplary embodiment the glass is formed into an optical focusingelement (a lens) or an optically transparent sealing cover to thecavity. The optically transparent top cover may further comprise aheating element associated and in contact with the cover.

The floor component material is characterized by (i) attenuation ofultrasonic energy of less than 8% per cm of the material, (ii) avelocity of shear waves mode ultrasound propagation that results inacoustical impedance such that the shear wave mode of the reference waveis less than 2% at the boundary of the floor component material andtransmission liquid medium, and (iii) reflection of a longitudinal modeof propagation for angles of incidence of greater than 60 degrees fromnormal to an interface with the ultrasound liquid transmission medium,whereby a velocity for longitudinal mode of greater than 1730 m/sec.

The transmission liquid medium may be water, wherein the floor componentmaterial has an ultrasonic shear wave impedance of from about 1170 toabout 1900 and an ultrasonic impedance (velocity times the density) ofgreater than 1200 but less than 2000. Such selected characteristics willresult in a shear wave mode reflection {(zS−ZW)/(zS+zW))̂2 where zS isthe impedance of the shear wave in the floor component and zW is theimpedance of water} of less than 5% at the floor component/water mediainterface. In a typical embodiment, the velocity for longitudinal modein the floor component is greater than 1730 and less than 2700 m/sec.

In one embodiment, the deformable detector material has vapor pressureof from about 1 torr to about 5 torr. In an exemplary embodiment, thedeformable detector material has a velocity of sound of less than 1,000m/Sec. The deformable detector material may be a fluorinated (in placeof hydrogen) organic compound having from 3 to 10 carbon atoms in astraight or branched chain or an aqueous solution with reduced surfacetension additives, wherein the viscosity of the detector deformabledetector material is such that the deformation will be formed on thedeformable detector material surface in less than 200 micro seconds andyet will be quiescent within less than 0.0083 or (1/120) seconds. In atypical application, the velocity of the detection deformable detectormaterial is approximately that of water (1497 m/sec) when surfacetension reduction additives are used.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 shows the state of the prior art illustrating the operation ofultrasonic holography (see U.S. Pat. No. 5,179,455 FIG. 3).Specifically, a plane wave of sound (1 a) (ultrasound) is generated bythe object (large area) transducer (1) (U.S. Pat. No. 5,329,202). Thesound is scattered (diffracted, refracted, etc.) by structural pointswithin the object (2). The scattered sound is from the internal objectpoints that lie in the focal plane (2 a) are focused (projected) intothe ultrasonic hologram plane (6) (U.S. Pat. No. 5,329,817). Thefocusing takes place by use of ultrasonic lens (3) (U.S. Pat. No.5,235,553) which focuses the scattered sound into a hologram detectorsurface (6) and the unscattered sound into a point (4). Since the focuspoint of the unscattered sound (4) is prior to the holographic detectorplane (6), this portion of the total sound again expands to form thetransparent image contribution (that portion of the sound thattransmitted through the object as if it were transparent orsemitransparent). In such an application, an ultrasound reflector (5) isused to direct the object sound at a different angle (preferablyvertically to allow for the holographic detector to have a surfaceparallel to ground to avoid gravity effects), thus impinging onhorizontal detector plane usually containing a deformable detectormaterial which is deformed by the ultrasound reflecting from thedeformable detector material-air interface. When the reference wave (7)and the object wave are simultaneous reflected from this detector, thedeformation of the deformable detector material-air interface is theexact pattern of the ultrasonic hologram formed by the object wave (1)and the “off-axis” reverence wave (7). The image produced by theapparatus of FIG. 1 (if no object present) is a completely white image.If there is a completely acoustically opaque object, the image will beblack.

FIG. 2 shows a side view of the inventive detector apparatus showing therequirement of a thick floor component (13) forming the base of a cavity(18) which has rigid side walls (14) and made of common plastic or metalconstruction materials. The cavity further contains a top element ofoptically transparent material (16). The side walls (14) are joined totop element (16) by a heat conducting fixture (15)

FIG. 3 shows an enlarged side view providing the angles of acoustic wavetransmission and propagation to the bottom surface of the floorcomponent, through the floor component, and through the detectiondeformable detector material surface medium. This figure shows thereflection of the longitudinal energy component (23) of the referencewave (21).

FIG. 3A shows an enlarged side view providing the angles of acousticwave transmission and propagation to the bottom surface of the floorcomponent, through the floor component, and through the detectiondeformable detector material surface medium. This figure shows that whenthe object wave (22 a) is at an angle (θ), other than perpendicular,there is generated both a shear wave (22 c) and longitudinal wave (22 b)component of the object wave.

FIG. 4 shows an additional inventive characteristic of the detectorfloor component that may contain ¼ wavelength matching layers on boththe top surface and the bottom surface of the floor component. Thisfeature will minimize reflections of the longitudinal waves entering thefloor component from the media into the floor component and that leavingthe floor component and entering the detector deformable detectormaterial.

FIG. 5 shows a top view of the inventive detector, floor component.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a long life sealed hologram detector unitthat is insensitive to vibrations in the area and provides improvedimage quality over previous hologram detectors. U.S. Pat. Nos.5,329,817; 5,585,847; 5,179,455; and 5,212,571 disclose an earlierembodiment for the forming an ultrasonic hologram on the surface of adeformable detector material resulting from the deformation of thesurface due to the reflection of the ultrasound energy profile of thecombination of the “object wave” that passes through the object and thatof the “reference wave” that is directed to the surface at an off axisangle from the “object wave”. The earlier embodiment required constantvigilance for unstable operations due to evaporation of the liquidmaterial used, and fogging of an optical lens used to view the formedhologram image. The present invention, by contrast, provides anapparatus having a specific combination and configuration of componentsfor an ultrasonic hologram detector or other ultrasonic imaging processthat accurately forms the phase and amplitude information of thehologram in a manner the renders the unit insensitive to environmentvibrations, capable of reflecting or transmitting coherent light toreconstruct images from the hologram, and provides long maintenance freefunctioning lifetime.

The present invention provides a hologram detector having a thick floorcomponent comprising a solid material having an upper surface forming afirst plane and a lower surface forming a second plane, wherein thefirst plane is essentially parallel to the second plane and wherein thedetector apparatus is positioned such that the first plane and secondplanes are essentially perpendicular to the force of gravity.

Floor Component

With regard to FIG. 2, the thick solid floor component (1) is made to beearth level and is made for a solid material. The thickness of the floorcomponent is from about 5 mm to about 7.5 mm. The functionalcharacteristics of the solid material are: i) minimal attenuation of theultrasound transmission, ii) a velocity of shear wave mode ultrasoundpropagation that results in an acoustical impedance being closelymatched with water or other commonly-used ultrasonic propagating media(e.g., aqueous solutions) (13), iii) a velocity of longitudinal wavemode propagation that will achieve full reflection for angles ofincident (Φ₁ of FIG. 3) of greater than 60 degrees from normal to aninterface with water. A preferred solid material is a rigid plastic havean impedance value for longitudinal sound energy waves such that thereflection at the water interface is not greater than 10% (e.g., 1490 to2880) and for shear wave propagation impedance such that the reflectionat the water interface is not greater than 3% (e.g., 1055 to 2120)Preferably, the solid floor material is selected from the groupconsisting of polycarbonate, PVC, polystyrene, cross-linked polystyrene,and polymethylpentene.

Further properties are to have water absorption of less than 0.1%, benonhazardous, contain no ingredients harmful to the environment, andhave a machinability such that dimension accuracy of 0.000254 cm can beobtained by grinding. An example of a floor component material that willmeet these specifications is cross-linked polystyrene. Preferably, thefloor component material has both top and bottom surfaces machined to aflatness of not more than +/−0.02 mm. The area of the floor component ispreferably in the shape of an rectangle with a “V” shaped end that ispositioned away from the direction of horizontal propagation of thereference ultrasonic energy. This shape contributes to the eliminationof residual surface sound waves that could distort subsequent imageswhen operating at a high frame rate (e.g., greater than 120 images persecond). The dimensions of the floor component is typically such thatthe illumination falls within the boundaries of the floor component andcan typically be of gross dimensions of 15 cm by 20 cm. The floorcomponent is machined such that there is a ridge around the exteriorboundary of the floor component of at least 2 mm in height. The purposeis to form a container within the cavity in which to hold deformabledetector material when the floor component is maintained in a planeperpendicular to the direction of earth gravity.

The inventive apparatus further comprises (FIG. 2) a rigid upper housing(20) that is attached to the component 15 that attaches the upperhousing to the side wall (14) of the detector which in turn is attachedto the raised edges of the thick solid flat floor component (13) butwhich provides an open cavity (18) above the floor of at least 1 cm butless than 5 cm in height. The thick solid flat floor component form atwo-dimensional rectangular in shape (when viewed from a top above thefloor or bottom below the floor) (FIG. 5) with an upper end of therectangle being “V” shaped and coming to a rounded point (25 of FIG. 4).The purpose of this shape is to have a place for any surface waves (26)on the imaging deformable detector material (19) to dissipate in therounded point which is placed away from the location and direct ofpropagation of the ultrasonic reference wave (7) from the referencetransducer.

In an alternative embodiment, FIG. 4 shows a detector floor componentthat may contain ¼ wavelength matching layers on both the top surfaceand the bottom surface of the floor component. This feature minimizesreflections of the longitudinal waves entering the floor component fromthe media into the floor component and that leaving the floor componentand entering the deformable detector material. The ¼ wave matching layerbetween the floor component is to be selected to have an ultrasonicimpedance (velocity times density) of (zB*zF)̂0.5, wherein zB is theultrasonic impedance of the base material and zF is the ultrasonicimpedance of the deformable detector material.

The matching layer between the floor component and the media is selectedto have an ultrasonic impedance (velocity times density) of (zB*zM)̂0.5,where zM is the ultrasonic impedance of the transmission media,preferably water. In both cases zB is chosen to be a number that isbetween the impedance for the longitudinal and shear wave modes,preferable mid range between these two values.

Cavity

FIG. 2 shows a side view of the inventive detector apparatus showing therequirement of a thick floor component (13) forming the base of a cavity(18) which has rigid side walls (14) and made of common plastic or metalconstruction materials. The rigid side walls are preferably from 1 cm to5 cm in height. The cavity further contains a top element of opticallytransparent material (16), preferably glass and may be an optical lens.The side walls (14) are joined to top element (16) by a heat conductingfixture (15), preferably metal and commonly aluminum or steel, on whichis contained a thermally attach a heating element (17). The heatingelement comprises a means for monitoring the temperature of the opticaltransparent element (16) and the temperature within the cavity (18).Most preferably, there is a computer controlled means to maintain thetemperature of the optically transparent top element (16) at atemperature of preferably 3 to 10° C. above the temperature of thecavity.

This apparatus is then preferably attached to a solid frame structure(20) that is part of the optical reconstruction subsystem of theultrasonic imaging system. The sealed cavity (18) is purged of air andfill to a positive pressure (preferably less than *50 torr) with aninert gas, for example, helium. Within the cavity, a deformable detectormaterial (19) is placed to cover the floor component to a thickness ofgreater than 0.2 mm but less than 0.5 mm. The detection deformabledetector material preferably is a fluorinated organic compound having asurface tension from about 12 to 19 dynes/cm and a Kinematic viscosityof from about 1 cs to about 20 cs but may be of water which has beentreated to reduce the surface tension.

When operational, the cavity (18) contains a thin layer of a detectiondeformable detector material (19). The layer of detection deformabledetector material is contained with the cavity. The detection deformabledetector material has a thickness greater than 0.2 and less than 0.5 mmand most preferably at a thickness that is a multiple of ¼ wavelengthsof the ultrasonic energy wave in the deformable detector material, atthe frequency being used in imaging. Preferably, the detectiondeformable detector material has a surface tension of from about 12dynes/cm to about 19 dynes/cm, a Kinematic viscosity of from about 1 csto about 20 cs, a vapor pressure of from about 1 torr to about 5 torr,and a velocity of sound of less than 1,000 m/Sec but may approach thanof water is a surface tension altering additive is used. Thesecharacteristics of the detection deformable detector material areimportant to the operation as the hologram pattern will build-up on thesurface within a reasonable time (e.g., less than 200 u sec) andmaintain the sound energy waveform for a sufficient time to performimaging (e.g., greater than 100 u sec) and yet be quiescent prior to thenext image period (typically within 8.3 m sec −120 frame/second rate).Preferably the detection deformable detector material is a fluorinatedorganic compound but may be water with a surface tension additives.

The cavity (18) is preferably sealed. It is preferably purged of air andreplaced with an inert gas. Preferably, the inert gas is selected fromthe group consisting of nitrogen, helium, argon, and combinationsthereof. This gas filled cavity is usually at atmospheric pressure but asmall positive pressure can be permitted.

The apparatus is made to form the bottom of a sealed cavity (18) the topof which is sealed by an optical lens or optical transparent cover (16).The lens or optical plate is contained in a metal collar (15) that ispart of the connective element between the hologram detector and theoptical reconstruction assembly. The optical lens or optical transparentcover may be but not required to be isolated from the metal housing by asealing gasket.

The thermal conducting collar of Item 15 incorporates an electricalheating element (17) placed on or in the collar (15) and containstemperature controls assembly (17 b) together with an electrical powersource (17 c) that controls the temperature of the optical lens oftransparent cover to a specified range. In the event this ring gets toocold (at a temperature below the main detector assembly), any outgassing and evaporation of the deformable detector material (if thematerial is liquid) in the detector will condense upon and cloud thelens or optical plate. In the event it gets too hot, the thermalgradients on the top of the deformable detector material surface willdistort and compromise the ability of the deformable detector materialsurface to form the hologram. Thus, a temperature of 3 to 10° C. aboveambient room temperature is maintained as the temperature of the opticallens or optical transparent cover. The controls of the heating elementare such that the temperature of the optically transparent cover will bemaintained at a higher ambient temperature than the interior of thecavity such that condensation of the optically transparent cover isminimized or eliminated. A temperature differential of approximately 6°C. is typical.

With regard to FIG. 3, the enlarged view shows the angles of ultrasonicenergy transmission from the deformable detector material transmissionmedia (preferably water) to the floor component and into the detectiondeformable detector material.

Wave Transmission

With regard to FIG. 3, the reference wave (21) is propagated toward thebottom surface of the floor component through a media (preferably water)and is incident upon the lower surface of the floor component at anangle Φ₁ with respect to the normal to the bottom surface of the floorcomponent. In general this angle Φ₁ is greater than 60 degrees. Uponstriking the bottom surface of the floor component, the reference wave(21) is divided into two components, namely the shear mode of ultrasonicwave propagation component (24) and the longitudinal wave mode ofpropagation component (23). The inventive process selects the materialof the floor component such that the longitudinal mode of the referencewave conversion will all be (or mostly be) reflected leaving only asingle reference wave (the shear wave mode portion) to enter the floorcomponent and thereby interfere with the object wave (22). This forms apure holographic pattern or other pure imaging pattern. To meet thischaracteristic, the longitudinal velocity in the floor component must begreater than the velocity in media/Sin Φ₁. For example, if the media iswater then the velocity in the media is approximately 1497 m/sec. Thusif Φ₁ is 60 degrees then the longitudinal velocity in the floorcomponent must be greater than 1497/0.866 or 1729 m/sec. Thus, when theincident angle (Φ) is greater than 60 degrees to the normal and thelongitudinal wave velocity in the floor component is greater than 1729m/sec, all of the longitudinal mode wave conversion (23) will bereflected at the interface between the media and the floor component andonly the shear wave mode (24) will enter the floor component andpropagate to the deformable detector material-air interface at the uppersurface of the floor component. This is important to the operation ofholographic imaging of subtle structures.

FIG. 3A illustrates another feature of this invention. When the objectwave (22) is directed to the floor component at an angle other thanperpendicular or near perpendicular angle, there is mode conversion toshear wave the present of which will compromise the purity and thus thequality of the hologram. Thus, another feature of this invention is toselect the solid material of the floor such that the amount of shearwave energy generated is less than a predetermined amount of totalobject wave energy when the angle of incidence of the object wave isless than a predetermined value. The amount of mode conversion intoshear wave energy as the object wave enters the lower surface of thedetector floor, prevents the hologram from being formed with two purewaves (one reference—usually shear wave mode—and one object—usuallylongitudinal wave mode). However in the case that the object wave entersthe floor at an angle other than perpendicular, there is generated two(not one) object wave components (one the longitudinal wave portion anda second shear wave energy). Thus this invention recognizes that theamount of energy conversion to shear wave mode is proportional to1−k*Cos((A Sin(v2/v1))*Sin(θ)), where k is an experimentally determinedfactor, v2 is the velocity of the longitudinal wave in the floor, v1 inthe velocity of the longitudinal mode in the media and θ is the initialangle of incidence for the object wave into the bottom of the floormaterial. It is advantageous to view an object at an angle with respectto the axis perpendicular to the lower surface of the floor. Thisinvention outlines the selection of floor material that will limit shearwave mode conversion of the object wave to an acceptable value. This iscarried out by selecting the maximum angle to the normal of the objectwave (e.g., 10 degrees) and the maximum amount of shear wave potentialenergy (e.g., 10%) and then selecting the material such that these twocriteria are met by experimental measurement.

An additional part of this invention is to design the floor materialsuch that the amount of reflected energy is below a predeterminedamount. If the ultrasonic impedance of the floor component is differentfrom the ultrasonic impedance of the media (which is true for mostpractical cases), there will be reflection at this interface. Includedin the inventive design is the selection of the floor component materialsuch that the ultrasonic impedance (velocity times density) for thefloor material is greater 1,800 but less than 2,500. This will result ina reflection of less than 6.5% at the floor component/water mediainterface for the longitudinal mode wave propagation into the floorcomponent from a water-based transmission media.

The detection material (19) is a deformable detector material. Onedeformable detector material can be a fluid and in this case, only alongitudinal wave mode is propagated from the arrival of the referencewave (21) and object wave (22). Also when the longitudinal wave mode ofpropagation in the detector deformable detector material (19) is slowerthan the shear wave mode velocity in the floor component, the incidentangle Φ₂ will be greater than Φ₃. One part of the inventive process isto select the characteristics of the detector deformable detectormaterial such that it has a surface tension that will allow the surfaceto respond to the wavelengths of ultrasound of 1 MHz to greater than 10MHz (e.g., 12 dynes/cm to about 19 dynes/cm), and a Kinematic viscosity(e.g., 1 cs to 20 cs) such that it will assume a stable condition afterthe ultrasound is turned off within 1/120 second or less and a thicknesssmall enough that it will not support horizontal waves of the frequencyencountered in buildings (e.g., less than 0.5 mm) and most preferably amultiple of ¼ wavelength in the deformable detector material at thefrequency being used in imaging.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

We claim:
 1. An apparatus comprising: (a) a rigid housing componentdescribing a cavity defined by a floor component composed of rigid solidmaterial having an upper surface forming a first plane and lower surfaceforming a parallel second plane, and rigid side elements attached to thebase, wherein the cavity defines an enclosed space of a dimension of theupper surface of the floor component and the rigid side elements of fromabout 1 cm to about 5 cm in height, wherein the distance between theupper surface and the lower surface of the floor component is from about5 mm to about 7.5 mm; (b) a layer of detection deformable detectormaterial contained with the cavity, wherein the detection deformabledetector material has a thickness from about 0.2 mm to about 0.5 mm;wherein the detection deformable detector material has a surface tensionof from about 12 dynes/cm to about 19 dynes/cm, wherein the detectiondeformable detector material has a Kinematic viscosity of from about 1cs to about 20 cs; and (c) an inert gas filling a space in the cavityabove the detection deformable detector material.
 2. The apparatus ofclaim 1 wherein the detection deformable detector material is afluorinated organic compound.
 3. The apparatus of claim 1 wherein theinert gas is selected from the group comprising of nitrogen, helium,argon, and combinations thereof.
 4. The apparatus of claim 1 wherein thethickness of the layer of detection deformable detector material is lessthan 0.254 mm.
 5. The apparatus of claim 4 wherein the thickness of thedetection deformable detector material is a multiple of ¼ wavelength ofthe ultrasound energy being used.
 6. The apparatus of claim 1 comprisingfurther a top of the cavity composed of optically transparent material.7. The apparatus of claim 6 wherein the optically transparent materialis glass.
 8. The apparatus of claim 7 wherein the glass is formed intoan optical focusing element or an optically transparent sealing cover tothe cavity.
 9. The apparatus of claim 6 wherein the opticallytransparent top cover further comprises a heating element associated andin contact with the cover.
 10. The apparatus of claim 1 wherein thefloor component material is characterized by (i) attenuation ofultrasonic energy of less than 8% per cm of the material, (ii) avelocity of shear waves mode ultrasound propagation that results inacoustical impedance such that a reflection shear wave mode is less than1.5% at the boundary of the floor component material and transmissiondeformable detector material medium, and (iii) reflection of alongitudinal mode of propagation for angles of incidence of greater than60 degrees from normal to an interface with the ultrasound deformabledetector material transmission medium, whereby a velocity forlongitudinal mode of greater than 1730 m/sec.
 11. The apparatus of claim10 wherein the transmission deformable detector material medium iswater, wherein the floor component material has an ultrasonic shear waveimpedance of from about 1170 to about
 1900. 12. The apparatus of claim10 wherein the velocity for longitudinal mode of the detectiondeformable detector material is approximately 2600 m/sec.
 13. Theapparatus of claim 1 wherein the detection deformable detector materialhas vapor pressure of from about 1 torr to about 5 torr.
 14. Theapparatus of claim 1 wherein the detection deformable detector materialhas a velocity of sound of less than 1,000 m/sec.
 15. An ultrasonicdetector apparatus to detect ultrasonic energy comprising: a floorcomponent composed of rigid material having an upper surface forming afirst plane and lower surface forming a parallel second plane; and rigidside elements attached to the floor component to define a cavity tocontain a layer of deformable detector material, wherein the material ofthe floor component is selected to have a velocity of longitudinal wavemode propagation that results in the reflection of a preselected amountof ultrasonic energy that would have otherwise become longitudinal wavemode propagation within the floor compartment, floor component materialhaving an ultrasonic impedance such that when incident ultrasonic energystrikes the lower surface at an angle of incidence exceeding apredetermined angle of incidence, the amount of shear wave energygenerated is less than a predetermined amount of the total wave energy.16. The apparatus of claim 15 wherein the lower surface is positionablein a media capable of propagating ultrasonic energy and a velocity ofpropagation of the longitudinal wave mode signals within the rigidmaterial of the floor component is greater than a velocity ofpropagation of ultrasound signals in the media divided by the Sin of anangle of incidence at which the ultrasound signals strike the floorcomponent lower surface.
 17. The apparatus of claim 15 wherein the rigidfloor component material is polymeric.
 18. The apparatus of claim 15wherein the lower surface of the floor component is positioned toreceive an object wave of ultrasonic energy at a first angle withrespect to the lower surface.
 19. The apparatus of claim 15 wherein thelower surface of the floor component is positioned to receive an objectwave of ultrasonic energy at a first angle with respect to the lowersurface and a reference wave of ultrasonic energy at a second angle withrespect to the lower surface, the second angle being greater than thepredetermined angle of incidence.
 20. The apparatus of claim 15 whereinthe predetermined angle of incidence for the second angle is ASin(Velocity of sound in Media/Velocity of sound in the floor component).21. The apparatus of claim 19 wherein the predetermined angle ofincidence is approximately 60 degrees.
 22. The apparatus of claim 15wherein the layer of deformable detector material is sufficiently thinas to not support standing waves less than approximately 500 Hz.
 23. Theapparatus of claim 15 wherein the layer of deformable detector materialis sufficiently thin as to not support standing waves typicallyencountered in building environments.
 24. The apparatus of claim 15wherein the layer of deformable detector material is from about 0.2 mmto about 0.5 mm thick.
 25. The ultrasonic hologram detector apparatus ofclaim 15 wherein the thickness of the deformable detector material is amultiple of ¼ wavelength of the ultrasonic energy being used.
 26. Theapparatus of claim 15 wherein the layer of deformable detection materialhas a surface tension of from about 12 dynes/cm to about 19 dynes/cm.27. The apparatus of claim 15 wherein the deformable detector materialis in a liquid state at typical room temperatures.
 28. The apparatus ofclaim 27 wherein the deformable detector material is a fluorinatedorganic compound.
 29. The apparatus of claim 15, further comprising aninert gas filling a space in the cavity above the deformable detectormaterial wherein the inert gas is selected from the group consisting ofnitrogen, helium, argon, and combinations thereof.
 30. The apparatus ofclaim 15, further comprising a top covering the cavity and comprising anoptically transparent material.
 31. The apparatus of claim 18 whereinthe floor component has a velocity of longitudinal wave mode propagationthat is sufficiently close to the velocity of sound in the media thatthe mode conversion of to shear wave energy (represented by 1−k*Cos(ASin (v2/v1)*Sin (first angle)) from the object wave entering the floorcomponent at a first angle is less than a predetermined percentage oftotal energy when the first angle of incident that is less than or equalto a predetermined angle.
 32. The apparatus of claim 31 wherein thepredetermined value of the first angle is 10 degrees.
 33. The apparatusof claim 31 wherein the percentage of shear wave propagation energy is10% of total object wave energy entering the floor component.
 34. Theapparatus of claim 30 wherein the optically transparent material isglass.
 35. The apparatus of claim 30 wherein the optically transparentmaterial is formed into an optical focusing element or an opticallytransparent sealing cover to the cavity.
 36. The apparatus of claim 30wherein the optically transparent top further comprises a heatingelement associated and in contact with the top.
 37. An apparatuscomprising: a floor component; first and second opposing side elementsattached to the floor component; a first endwall coupled to the floorcomponent and the first and second sidewalls; and a V-shaped secondendwall coupled to the floor component and the first and secondsidewalls and opposing the first endwall, the apparatus being positionedto receive a reference wave having a horizontal direction of propagationtoward the V-shaped endwall, wherein the floor compartment, first andsecond sidewalls and first and second endwalls define a cavity, theapparatus further comprising a layer of deformable detector materialcontained within the cavity.
 38. The apparatus of claim 37, furthercomprising an inert gas filling a space in the cavity above thedeformable detector material wherein the inert gas is selected from thegroup consisting of nitrogen, helium, argon, and combinations thereof.39. The apparatus of claim 37, further comprising a top covering thecavity and comprising an optically transparent material.
 40. Theapparatus of claim 39 wherein the optically transparent material isglass.
 41. The apparatus of claim 39 wherein the optically transparentmaterial is formed into an optical focusing element or an opticallytransparent sealing cover to the cavity.
 42. The apparatus of claim 39wherein the optically transparent top further comprises a heatingelement associated and in contact with the top.